A yeast is a special type of "budding" fungus. Candida albicans is a common yeast organism that causes yeast infections in humans. The term "fungus" applies to yeasts as one group of the fungus kingdom (Mycota). Molds are the other organisms which belong to the fungus kingdom.
How to tell a yeast from a mold fungus
Yeasts have been known ever since the days of making and baking bread. Yeast, when added to flour, causes the flour to rise, hence the term "leavened" bread. This leavened bread could more accurately be called "yeast" bread.
Yeasts bud. When they reproduce, they blow out a small part of their wall, like a balloon, and expand that bud into another yeast, which eventually separates and falls away. Molds are fungi. Fungi produce filaments or threads called "hyphae." In summary, yeasts bud, but moulds form filaments, or hyphae.
How does a yeast infection really differ from a mold infection?
Typically, Candida albicans is the biggest culprit in yeast infections. Candida causes infections of mucocutaneous membranes of the mouth, gastrointestinal tract, and the genitalia. The yeasts reproduce rapidly by budding, and are enriched in their growth by carbohydrates (sugars and starches) in the body tissues. Fungi of the mould group often cause different types, signs and symptoms of infectious disease.
Yeast infection and mold infection defined further
Infection in the classic definition means that a microorganism, such as a yeast or mould, has entered into or onto the body. A person can become infected with a microbe and not become diseased. When the microbe---yeast, mould, bacterium, virus---begins to multiply in the infected site, then there is actual disease. So, a true yeast infection, in most cases, is best termed "yeast disease," but by common acceptance when someone has a yeast infection it is understood that they have a diseased state that must be treated.
Treating infections of yeast and molds
There are antibiotics that can be used to treat yeast and mould infections. These fungal antibiotics are different from those used for bacterial infections.
A yeast infection is a fungus infection, not a mold infection
Finally, when someone says they have a yeast infection---that is clear. When someone says they have a fungus infection, you can ask, "Is it a yeast, or a mould?" That question is clarified here.